1. Field of the Invention
The present invention relates to a liquid crystal light valve apparatus, and in particular to a liquid crystal light valve apparatus which improves a contrast to thereby improve an image quality.
2. Description of the Related Art
A liquid crystal light valve (LCLV) apparatus is an optical to optical image converter. A light valve is such an apparatus adapted to being capable of receiving a light which is low in light intensity, reading out its optical image on real time by using a light fed from another light source, and output the image. Hereafter, a liquid crystal light valve apparatus is also referred to simply as a liquid crystal light valve.
The liquid crystal light valves are classified into a transmission type and a reflection type.
In the reflection type, there is used an active matrix scheme in which a signal is supplied from an output terminal to each pixel to turn on or off each pixel, and a light writing-in scheme in which light writing is conducted on a photoconductive layer from a rear face side opposite to its output.
On the other hand, in the transmission type, only the active matrix scheme is used.
The fact common to the above described three schemes is that a liquid crystal layer is interposed between opposed electrodes.
In order to make birefringence of the liquid crystal layer in these liquid crystal light valves constant in a pixel area, therefore, the spacing between both substrates having electrodes opposed to each other via the liquid crystal layer needs to be kept constant.
For the purpose of keeping the spacing between the substrates constant, a spacer is provided.
Heretofore, a method shown in FIG. 1 has been used. On a first substrate 51 having function elements such as a transistor or the like formed thereon, globular beads 53 having uniform diameters are scattered. A second substrate 52 having an opposed electrode formed thereon is stuck to the scattered beads 53.
In this method, however, the beads 53 serving as the spacer are scattered also on pixel electrodes 55. Therefore, shadows of the beads 53 appear in a projected image. In addition, random distribution of the shadows of the beads 53 causes conspicuousness. This results in a problem of degraded image quality.
In order to eliminate the shadows of the beads 53, therefore, there is widely used such a method that the spacers 53 are not disposed in the image display area. According to the method, when adhering the peripheral portions of the substrates, spacers are added to the bonding agent thereof t o form the spacing.
In this case, the surface flatness of both substrates on the side contacting the liquid crystal layer needs to be limited to, for example, 0.3 .mu.m or less over the whole image display area.
As a matter of fact, however, a glass substrate, a semiconductor substrate or the like having a thickness of 1.1 mm or less widely used as a liquid crystal glass substrate is easily bent by stress of a thin film electrode layer such as metal or the like, a semiconductor film such as amorphous silicon or the like, and an insulation film or the like formed thereon.
Therefore, the above described method of forming the spacer only around the periphery of glass has been put to practical use only in the case where the liquid crystal light valve apparatus is small with a surface area comprised of the above described pixel display area and a peripheral portion thereof being equal to or less than 30 mm in each side length and in addition the error allowance of the substrate spacing is comparatively large, such as in the case of the so-called twist pneumatic oriented normally white mode of transmission type.
In other words, in the case where the error allowance of the spacing is small as in a liquid crystal light valve having many pixels and a large display area, a liquid crystal light valve of reflection type using an electrically controlled birefringence (ECB) mode and so on, it is difficult to apply the above described method of forming the spacers only in the periphery of glass and it becomes necessary to dispose spacers in the display area.
In order to dispose the spacers so as not to make the shadows of the spacers inconspicuous, therefore, it is made necessary to dispose the spacers selectively only the portion between pixels.
And there is being studied a method of forming a so-called pillar-shaped spacer 54 by using registering a as self alignment with respect to pixel electrodes 55 forming pixels, for example, as shown in FIG. 2.
FIG. 3 shows a schematic sectional view of a conventional liquid crystal light valve of transmission type.
This liquid crystal light valve of transmission type has the following configuration. A first substrate 51 is formed by forming pixel electrodes 55 divided so as to be associated with respective pixels, on an internal surface of a base substrate 50 made of, for example, glass and forming an orientation film 61 over the entire internal surface so as to cover the pixel electrodes 55. A second substrate 52 is formed by forming opposed electrodes 60 on an internal surface of a base substrate 50 made of, for example, glass and forming an orientation film 62 over the entire internal surface so as to cover the opposed electrodes 60. The first substrate 51 and the second substrate 52 are disposed so as to be opposed to each other via pillar-shaped spacers 54. In addition, the peripheral portions of the substrates 51 and 52 are hermetically sealed, and a liquid crystal layer 58 is formed between the substrates 51 and 52. The liquid crystal light valve of transmission type has thus been formed.
If in FIG. 3 the pixel electrodes 55 of the first substrate 51 are made of a material having a high light reflection factor such as Al, Cr, W or the like, or alternatively a dielectric multi-layer reflection film or the like is disposed between the pixel electrodes 55 and the liquid crystal layer 58, then a liquid crystal light valve of reflection type is directly obtained.
Especially, in order to sink the wholly black state, i.e., make the wholly black state blacker in the liquid crystal light valve of FIG. 3, for example, a polarization plate 56 and a analyzer plate 57 are respectively disposed on the first and second substrates 51 and 52 so as to satisfy the orthogonal Nicol relation. Directions of the orientation films 61 and 62 respectively provided on the pixel electrodes 55 and the opposed electrodes 60 which are in turn provided on opposed sides respectively of the substrates 51 and 52 are made the same as those of the polarization plate 56 and the analyzer plate 57, respectively. Between the orientation films 61 and 62, a liquid crystal is injected.
Operation of this liquid crystal light valve is shown in FIGS. 4A and 4B.
In a state of FIG. 4A in which any voltage is not applied between both the electrodes 55 and 60 in the above described configuration, incident light 64 applied to the liquid crystal panel passes through the polarization plate 56, becomes a linearly polarized light 65, optically rotates along the twist of liquid crystal molecules 59, and passes through the analyzer plate 57 as it is. This results in a bright state.
On the other hand, in a state of FIG. 4B in which a voltage is applied between the electrodes 55 and 60, the liquid crystal molecules 59 are oriented vertically. Irrespective of its wavelength, therefore, the linearly polarized light 65 through the polarization plate 56 cannot pass through the analyzer plate 57 disposed perpendicular to the polarization plate 56. As a result, the wholly black state can be implemented.
The configuration conducting the above described operation is in the bright state when no voltage is applied, and hence its operation mode is called a normally white mode. Since the wholly black state can be implemented irrespective of the light wavelength, this configuration recently tends to be adopted in many cases.
When the above described spacers 54 are disposed between the first and second substrates 51 and 52, and the liquid crystal molecules 59 are vertically oriented in the wholly black state, liquid crystal molecules 63 located around the pillar-shaped spacers 54 are subject to intermolecular attraction between them and the pillar-shaped spacers 54 and their orientation directions are disturbed as shown in the sectional view of FIG. 3.
At this time, light 67 passed through the vicinity of the pillar-shaped spacers 54 is subject to birefringence due to the liquid crystal molecules 63 deviated from the vertical orientation. As a result, the polarization direction of the light 67 is partially rotated. A part of the light 67 thus passes through the analyzer plate 57, resulting in a light leak 68.
As the liquid crystal light valve, therefore, the wholly black display cannot be made completely black. As a result, the contrast is significantly lowered. A countermeasure against this is needed.
In an alternative configuration of the liquid crystal light valve of reflection type, a liquid crystal of negative type is used and consequently the liquid crystal molecules are oriented perpendicular to the substrates when no voltage is applied between the electrodes 55 and 60. Furthermore, polarization beam splitters (PBS) may be used instead of the polarization plate 56 and the analyzer plate 57.
This PBS has a function of reflecting only a certain polarized light component of the incident light and passing other components through it. FIG. 5 is a schematic diagram of an optical system formed by using a liquid crystal light valve of reflection type. As shown in FIG. 5, a liquid crystal light valve 25, a light source 24, and a projection optical system 22 are disposed around the PBS.
The case where a voltage is applied in this state between the electrodes 55 and 60 (see FIG. 3) having a liquid crystal layer interposed between will now be described. A light emitted from the light source 24 and passed through a lighting optical system 28 becomes a linearly polarized light, such as, for example, a light having a polarization direction perpendicular to paper of the drawing by being passed through the PBS 23, and the n irradiated on the liquid crystal light valve 25 to enter the liquid crystal layer 58. Although not illustrated, the liquid crystal molecules 59 of the liquid crystal layer 58 (see FIG. 3) are inclined from the vertical orientation to a direction parallel to the substrate surface by the application of the voltage at this time.
At this time, the incident light is passed through the inside of the liquid crystal layer 58 and reflected on a pixel surface. Or in the light writing scheme, the incident light is reflected on a reflection film such as a dielectric reflection film or the like provided on the pixel surface. As a result, the reflected light is passed through the liquid crystal layer 58 again. Since the liquid crystal molecules 59 are inclined, the reflected light is influenced by the birefringence during this time. As a result, there is obtained an output light having a polarization state different from that of the incident light which is linearly polarized light, such as, for example, light having a component in a direction perpendicular to the paper of the drawing and a component in a lateral direction.
When this light enters the PBS 23 again, a light component 31 changed in polarization state, such as, for example, only the component in the lateral direction is passed through the PBS 23 and the projection optical system 22, and finally led to a projection screen 21.
On the other hand, if the voltage is not applied between both the electrodes, then the liquid crystal molecules remain in the vertical orientation. Therefore, the incident light is reflected in the liquid crystal light valve 25 while remaining linearly polarized, put out from the liquid crystal light valve apparatus 25, and returned to the PBS 23. However, this light is not influenced in the liquid crystal layer 58 by the above described birefringence.
Therefore, the direction of the linearly polarized light of the incident light is not changed. As a result, the light returned to the PBS 23 is returned to the light source 24 as it is. Since there is no light entering the projection optical system 22, black display is incurred. This is called the normally black mode.
At this time, intermolecular attraction is exerted between the liquid crystal molecules located in the vicinity of the pillar-shaped spacer 54 and the pillar-shaped spacer 54. Therefore, the liquid crystal molecules located in the vicinity of the pillar-shaped spacer 54 are inclined from the vertical orientation which is the original orientation of the liquid crystal to the horizontal direction which is parallel to the substrates 51 and 52. Thereupon, the light passed through this area, such as, for example, the linearly polarized light is subjected to birefringence in the liquid crystal layer. When the light goes through and returns to the liquid crystal layer 58 and leaves the liquid crystal light valve apparatus 25, its polarization has already been changed. When this light has entered the PBS 23, a light of a component different from the original linearly polarized light is passed through the PBS 23, and projected onto the screen 21 via the projection optical system 22. In the same way as the above described configuration of the normally white mode, the so-called light leak is caused.
In other words, on a picture on the screen 21 which should originally be wholly black, vicinities of the pillar-shaped spacers 54 shine. As a result, the contrast ratio of the wholly black state to the maximum luminance is lowered and the quality of video images is degraded.
This problem appears significantly especially in a liquid crystal light valve of reflection type improved in light utilization factor by putting the pixel electrodes close together and thereby reducing invalid areas, when regulation force for obtaining the vertical orientation is received from the orientation film and appears remarkably when the regulation force does not depend upon the electric field.
Whether the configuration is that of the normally white mode or that of the normally black mode, therefore, there is a problem that the light leak is caused by the disturbance of orientation of the liquid crystal molecules in the vicinity of the pillar-shaped spacers.